1. Field of Invention
The present invention is generally directed to the field of internal combustion engine monitoring and control systems and specifically to a method and system for monitoring cyclic variability in reciprocating engines.
2. Description of Prior Art
Reciprocating engines (also known as piston engines) compress and burn a quantity of fuel/air mixture in each cylinder during each engine cycle. A cycle is completed during one crankshaft revolution for two-stroke engines and takes two crankshaft revolutions for four-stroke engines. Within any given cylinder, successive cycles will exhibit variations in how quickly a given fraction of the fuel/air mixture is burned. This cycle to cycle variation is commonly referred to as cyclic variability and has long been recognized as a fundamental characteristic of reciprocating engines.
Cyclic variability impacts the torque and speed stability of the engine as well as the balance of load sharing between cylinders, the engine vibration levels and exhaust emissions produced by the engine. Therefore, cyclic variability must be limited to acceptable levels. The practical limits for cyclic variability depend upon the engine application.
Situations where some cycles have little or no combustion (known as misfiring) create high cyclic variability levels, and are unacceptable in almost all situations. Thus misfire detection may be considered as a rudimentary form of cyclic variability monitoring. However, it is much more desirable to be able to monitor lower levels of cyclic variability in situations where all of the cycles have complete or nearly complete. With this degree of engine monitoring capability, operation within acceptable cyclic variability limits could be maintained and misfiring could be avoided.
The most successful prior art schemes which have been used to monitor cyclic variability include in-cylinder pressure sensing, spark plug ion current sensing and instantaneous crank angle velocity sensing.
Drawbacks of in-cylinder pressure sensing include the need for a passage into the cylinder to mount the sensor, relatively high cost and limited durability.
Spark plug ion current sensing does not provide good sensitivity to low levels of cyclic variability and it cannot be used in engines without spark plugs such as diesel, dual fuel and gas diesel engines. It has also been shown that the achievement of good ion sensing performance in lean burn engines requires ignition system modifications such as short spark duration, large electrode area and shrouded spark plugs. These modifications tend to degrade the ability of the ignition system to ignite lean mixtures reliably.
Instantaneous crank angle velocity sensing systems do not provide individual signals from the individual cylinders of multi-cylinder engines. This limits the ability of these systems to detect low levels of cyclic variability. The crank angle velocity measurements of these systems are also susceptible to mechanical perturbations such as shocks and vibrations and are affected by crankshaft tensional behavior.
The use of exhaust gas temperature measurements in engine monitoring systems is well known in prior art. Sheathed thermocouples are the most commonly used type of exhaust temperature sensor. It is common for each cylinder of large stationary engines to be equipped with an exhaust temperature thermocouple located near each exhaust port. These thermocouples are inexpensive, durable and easy to install. However, conventional exhaust thermocouples have slow transient response to charges in temperature, because they must be large enough to provide adequate durability. Consequently, the signals from these sensors can only provide an indication of the time-averaged exhaust temperature value over many engine cycles. These slow response temperature signals can be used to detect severe cylinder faults such as continuous misfiring by comparing the signal values to predetermined thresholds or current average values for all of the engine cylinders. U.S. Pat. Nos. 3,939,711 and 4,122,720 describe examples of this type of system. None of these prior art approaches to exhaust temperature monitoring have provided cycle-by-cycle information suitable for cyclic variability monitoring.
What is needed is a means of monitoring cyclic variability that combines the desirable attributes of exhaust port thermocouples (durable, inexpensive, easily installed, and providing an isolated signal for each cylinder) with the ability to measure low levels of cyclic variability like in-cylinder pressure sensors. In view of the foregoing, the primary object of the invention is to provide a method for monitoring cyclic variability in a reciprocating engine by analyzing exhaust gas temperature sensor signals. As used herein, the term cyclic variability is interchangeable with other commonly used terms including cyclic variation, combustion variation, combustion variability, cyclic dispersion, combustion instability and engine instability. The cycle-to-cycle variations that are monitored can include differences between cycles with complete or nearly complete combustion and the detection of cycles with abnormal combustion such as misfire and incomplete burning. The cyclic variability information obtained by the invention has uses which include providing feedback signals for an engine control system, providing input signals for an engine diagnostic system, and providing input signals for an engine instrument panel.